Talk In Code

The isNaN() function (NaN stands for Not a Number) can be useful if you are looking at form inputs or similar and is used to detect if a value is not a number. For example, the following code shows the output of isNaN() on two variables.

var number42 = "42";
var wibble = "wibble";
alert(isNaN(number42)); // Prints out false
alert(isNaN(wibble)); // Prints out true


This first test is true because the number 42 is, in fact, a number. The second test is false because wibble isn’t a number. This is simple enough, but what if we started looking at some different values?

var empty = "";
var nothing = null;
alert(isNaN(empty)); // Prints out false
alert(isNaN(nothing)); // prints out false

What is happening here is that isNaN() tries to converts any value it is given into an number, which means that "" and null get converted to 0 and the function returns false because they are now numbers. You can try this out yourself by doing one of the following:

var empty = "";
alert(+empty); // Prints out 0
alert(Number(empty)); // Prints out 0

To solve this problem you can use the parseInt() function. This takes a string as an input and tries to convert it into a number, if this is not possible then it returns NaN. If parseInt() is given an empty string or a null value it will also return NaN, rather than converting them to 0. This gives us a mechanism by which we can test values using a combination of parseInt() and isNaN().

var empty = "";
alert(isNaN(parseInt(empty))); // Prints out true


Finally, if we give isNaN() and parseInt() an undefined value the results are as follows.

var nothing;
alert(parseInt(nothing) + ", " + isNaN(nothing)); // Prints out NaN, true


The isNaN() function returns true because it can’t convert the value into any number.

var nothing;
alert(+nothing);
alert(Number(nothing));

This code prints out NaN for both of the alert statements.

You think I’m joking right? Well due to a silly mistake when creating the language the concatenation character is the plus symbol. The same symbol used when you are adding numbers together and so sometimes JavaScript will add them together and sometimes it will concatenate them.

This occurs if JavaScript encounters any part of the calculation to be a string. If it is then it will concatenate it. For example.

alert("1"+2+3);// prints out "123" rather than 6

To stop this you need to add the parseInt() function to the part of the addition that might be mistaken as a string. Or the whole thing just to make sure. The following is a bit of an overkill but ensures that the values will be added, NOT concatenated.

alert(parseInt(1)+parseInt(2)+parseInt(3));

It is especially important to do this if you get any values from any form fields. This is because they are passed to JavaScript as strings so you will need to parseInt() the values to work with them.